In addition to magnetic resonance tomography (MR), positron emission tomography (PET) is also being increasingly used in medical diagnostics. While MR is an imaging method for showing structures and slices inside the body, PET allows in vivo visualization and quantification of metabolic activities.
PET uses the particular properties of positron emitters and positron annihilation in order to quantitatively determine the function of organs or cell areas. With this technique, the patient is administered appropriate radiopharmaceuticals marked with radionuclides prior to the examination. As they decay, the radionuclides emit positrons which after a short distance interact with an electron, causing what is termed annihilation to occur. This results in two gamma quanta which fly apart in opposite directions (offset by 180°). The gamma quanta are detected by two opposed PET detector modules within a particular time window (coincidence measurement), by means of which the annihilation site is localized to a position on the line connecting said two detector modules.
In the case of PET, the detector module must generally cover the greater part of the gantry arc length for the purpose of detection. It is subdivided into detector elements having a side length of a few millimeters. On detecting a gamma quantum, each detector element generates an event record that specifies the time and the detection location, i.e. the corresponding detector element. These items of information are transmitted to a fast logic unit and compared. If two events coincide within a maximum time period, it is assumed that there is a gamma decay process on the connecting line between the two associated detector elements. The PET image is reconstructed using a tomography algorithm, i.e. so called back projection.
For PET examinations, measurement data is typically obtained by several hundred detector elements in a precisely timed manner. Only events which are detected simultaneously within a time window by two sensors are actually evaluated. In PET scanners, the signals are digitized and mathematically evaluated close to the detector elements.
In combined MR/PET scanners, the PET gantry must be incorporated close to the patient port of the MR/PET equipment, thereby further exacerbating the space problems to be solved anyway with MR scanners. It is therefore desirable to incorporate as few PET unit components as possible in the PET gantry.
Moreover, because of the high static magnetic field required for MR examinations, an evaluating computer must be a certain minimum distance away. In addition, one or more signal processing units, for example, may be disposed outside the PET gantry and even outside the actual MR/PET device. The signals of the detector elements must then be fed out to the signal processing unit(s) via signal lines. Consequently, a plurality of connecting lines to an evaluating signal processing unit are required for evaluating and detecting the signals of the detector elements. This must be implemented in as space-saving a manner as possible, i.e. using as few signal lines as possible.
It is basically possible for digitized components to be incorporated close to the detection unit in the MR tester, for which e.g. fiberoptic transmission of the signals to the evaluating computer is possible. However, interference with the MR system by the RF components required for this purpose cannot be eliminated, resulting in image artifacts in the MR system.
Crystals which can detect several events are frequently used as detector elements. These are structured, for example, as a 3×3 matrix. Here nine detection units are therefore combined to form one detector element. With an arrangement of this kind it is possible for the nine detection units to be read out using a reduced number of signal lines. This reduction in the signal lines is possible through suitable analog calculating of the signals of the detection units. So-called Anger logic is frequently used for the calculation, in which the barycentric coordinates (X, Y) of the scintillation in the detector and its summed energy are determined in an analog manner and transmitted. Only 3 signal lines (and ground connection) are therefore required for the 3×3 matrix (or other detector arrangement). This has been disclosed by Karp et al. in “Performance of a Brain PET Camera Based on Anger-Logic Gadolinium Oxyorthosilicate Detectors”, Journal of Nuclear Medicine, Vol. 44 No. 8, (2003), 1340-1349, the entire contents of which is hereby incorporated herein by reference.
In RF technology it is already known to connect a plurality of signal sources to a plurality of amplifier elements via a switching matrix. The switching matrix used comprises a number of intersecting transmission lines which can be multiply used by switching elements at their points of intersection, said number corresponding to the number of signal sources and amplifier elements. The matrix-like structure enables each signal source to be connected to each amplifier element. This enables the number of signal lines to be significantly reduced compared to implementing the connection between the components by means of individual signal lines. An arrangement of this kind is disclosed, for example, in DE 10 2004 055 939 B4, the entire contents of which is hereby incorporated herein by reference.